By Noemie Olive and Sudip Kar-Gupta PARIS (Reuters) -French workers went on strike and joined marches across the country on Thursday, halting trains and cutting electricity production in protest against government plans to raise the retirement age by two years to 64. The stoppages are a major test for President Emmanuel Macron, who said on Thursday that his pension reform plan, which opinion polls show is hugely unpopular, was “just and responsible” and had to be carried out. Protesters disagreed. “It’s salaries and pensions that must be increased, not the retirement age,” read one large banne…